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Volumetric, method.—In this method the apparatus measures the average daily concentration of
sulphur dioxide. Air from outside a building is drawn at the rate of about 50 cubic feet per day
through a bubbler. The bubbler contains a dilute solution of hydrogen peroxide, and sulphur dioxide
from the air is absorbed in the solution to form sulphuric acid. At the end of the day's run the volume
of the standard alkali needed to neutralize the sulphuric acid is determined and hence the mean
concentration of sulphur dioxide is calculated.
The mean concentration of sulphur dioxide at Golden Lane during 1949 was 0.09 parts of sulphur
dioxide per million parts of air. Corresponding values for other stations in Great Britain making this
type of observation are as follows : Westminster 0.09, Bilston 0.10, Crossness 0.03. Figure 3 records
the average monthly concentrations of sulphur dioxide during 1949 at these four stations.
Fig.3 Monthly variation of sulphur dioxide, 1949.
(Volumetric method).
The instrument at Crossness is in open country about eleven miles east of Charing Cross, Barking
Power Station being 1½ miles to the W.N.W. The instrument at Bilston (in the "Black Country"
area) is surrounded by a district which is residential and industrial.
In the winter of 1938-39 the average concentration of sulphur dioxide in eight London Districts
was 0.13 parts per million by volume.
It will be noticed that marked variations occur in the concentration of sulphur dioxide. This
concentration is affected to a considerable extent by meteorological factors such as turbulence and
rain as a result of which the variations observed are considerably greater than the changes in the
rate of emission of sulphur dioxide. There is also a marked seasonal variation of a type to be expected,
i.e., a peak in the winter months, and a minimum in the summer.
The maxima and minima are high for central London. This is probably due to the large number
of office and domestic fires in the neighbourhood. Comparing the City with the manufacturing district
of Bilston it will be seen that although the summer concentration in Bilston is higher, the winter
peak is lower than in the City. This suggests that it is the domestic, rather than the industrial,
chimneys which are responsible for the large seasonal variation.
Lead Peroxide Method.—This method of estimating atmospheric sulphur dioxide was designed
primarily to provide a measure of the "activity" of attack of sulphur pollution on the fabric of
buildings. It is based on the facility with which gaseous sulphur dioxide combines with lead peroxide
to form lead sulphate.